In the fabrication of semiconductor devices from a silicon wafer, a variety of semiconductor processing equipment and tools are utilized. One of these processing tools is used for polishing thin, flat semiconductor wafers to obtain a planarized surface. A planarized surface is highly desirable on a shallow trench isolation (STI) layer, inter-layer dielectric (ILD) or on an inter-metal dielectric (IMD) layer, which are frequently used in logic & memory devices. The planarization process is important since it enables the subsequent use of a high-resolution lithographic process to fabricate the next-level circuit. The accuracy of a high resolution lithographic process can be achieved only when the process is carried out on a substantially flat surface. The planarization process is therefore an important processing step in the fabrication of semiconductor devices.
A global planarization process can be carried out by a technique known as chemical mechanical polishing, or CMP. The process has been widely used on ILD or IMD layers in fabricating modern semiconductor devices. A CMP process is performed by using a rotating platen in combination with a polishing head. The process is used primarily for polishing the front surface or the device surface of a semiconductor wafer for achieving planarization and for preparation of the next level processing. A wafer is frequently planarized one or more times during a fabrication process in order for the top surface of the wafer to be as flat as possible. A wafer can be polished in a CMP apparatus by being placed on a carrier and pressed face down on a polishing pad covered with a slurry of colloidal silica, CeO2 or aluminum.
A polishing pad used on a rotating platen is typically constructed in two layers overlying a platen, with a resilient layer as an outer layer of the pad. The layers are typically made of a polymeric material such as polyurethane and may include a filler for controlling the dimensional stability of the layers. A polishing pad is typically made several times the diameter of a wafer in a conventional rotary CMP, while the wafer is kept off-center on the pad in order to prevent polishing of a non-planar surface onto the wafer. The wafer itself is also rotated during the polishing process to prevent polishing of a tapered profile onto the wafer surface. The axis of rotation of the wafer and the axis of rotation of the pad are deliberately not collinear; however, the two axes must be parallel. It is known that uniformity in wafer polishing by a CMP process is a function of pressure, velocity and concentration of the slurry used.
A CMP process is frequently used in the planarization of an STI, ILD or IMD layer on a semiconductor device. Such layers are typically formed of a dielectric material. A most popular dielectric material for such usage is silicon oxide. In a process for polishing a dielectric layer, the goal is to remove typography and yet maintain good uniformity across the entire wafer. The amount of the dielectric material removed is normally between about 2000 A and about 20,000 A. The uniformity requirement for ILD or IMD polishing is very stringent since non-uniform dielectric films lead to poor lithography and resulting window-etching or plug-formation difficulties. The CMP process has also been applied to polishing metals, for instance, in tungsten plug formation and in embedded structures. A metal polishing process involves a polishing chemistry that is significantly different than that required for oxide polishing.
Important components used in CMP processes include an automated rotating polishing platen and a wafer holders which both exert a pressure on the wafer and rotate the wafer independently of the platen. The polishing or removal of surface layers is accomplished by a polishing slurry consisting mainly of fumed, colloidal silica or CeO2 suspended in deionized water or alkali solution. The slurry is frequently fed by an automatic slurry feeding system in order to ensure uniform wetting of the polishing pad and proper delivery and recovery of the slurry. For a high-volume wafer fabrication process, automated wafer loading/unloading and a cassette handler are also included in a CMP apparatus.
As the name implies, a CMP process executes a microscopic action of polishing by both chemical and mechanical means. While the exact mechanism for material removal of an oxide layer is not known, it is hypothesized that the surface layer of silicon oxide is removed by a series of chemical reactions which involve the formation of hydrogen bonds with the oxide surface of both the wafer and the slurry particles in a hydrogenation reaction; the formation of hydrogen bonds between the wafer and the slurry; the formation of molecular bonds between the wafer and the slurry; and finally, the breaking of the oxide bond with the wafer or the slurry surface when the slurry particle moves away from the wafer surface. It is generally recognized that the CMP polishing process is not a mechanical abrasion process of slurry against a wafer surface.
While the CMP process provides a number of advantages over the traditional mechanical abrasion type polishing process, a serious drawback for the CMP process is the difficulty in controlling polishing rates at different locations on a wafer surface. Since the polishing rate applied to a wafer surface is generally proportional to the relative rotational velocity of the polishing pad, the polishing rate at a specific point on the wafer surface depends on the distance from the axis of rotation. In other words, the polishing rate obtained at the edge portion of the wafer that is closest to the rotational axis of the polishing pad is less than the polishing rate obtained at the opposite edge of the wafer. Even though this is compensated for by rotating the wafer surface during the polishing process such that a uniform average polishing rate can be obtained, the wafer surface, in general, is exposed to a variable polishing rate during the CMP process.
Recently, a chemical mechanical polishing method has been developed in which the polishing pad is not moved in a rotational manner but instead, in a linear manner. It is therefore named as a linear chemical mechanical polishing process, in which a polishing pad is moved in a linear manner in relation to a rotating wafer surface. The linear polishing method affords a more uniform polishing rate across a wafer surface throughout a planarization process for the removal of a film layer from the surface of a wafer. One added advantage of the linear CMP system is the simpler construction of the apparatus, and this not only reduces the cost of the apparatus but also reduces the floor space required in a clean room environment.
A typical conventional CMP apparatus 90 is shown in FIG. 1 and includes a base 100; polishing pads 210a, 210b, and 210c provided on the base 100; a head clean load/unload (HCLU) station 360 which includes a load cup 300 for the loading and unloading of wafers (not shown) onto and from, respectively, the polishing pads; and a head rotation unit 400 having multiple polishing heads 410a, 410b, 410c and 410d for holding and fixedly rotating the wafers on the polishing pads.
The three polishing pads 210a, 210b and 210c facilitate simultaneous processing of multiple wafers in a short time. Each of the polishing pads is mounted on a rotatable carousel (not shown). Pad conditioners 211a, 211b and 211c are typically provided on the base 100 and can be swept over the respective polishing pads for conditioning of the polishing pads. Slurry supply arms 212a, 212b and 212c are further provided on the base 100 for supplying slurry to the surfaces of the respective polishing pads.
The polishing heads 410a, 410b, 410c and 410d of the head rotation unit 400 are mounted on respective rotation shafts 420a, 420b, 420c, and 420d which are rotated by a driving mechanism (not shown) inside the frame 401 of the head rotation unit 400. The polishing heads hold respective wafers (not shown) and press the wafers against the top surfaces of the respective polishing pads 210a, 210b and 210c. In this manner, material layers are removed from the respective wafers. The head rotation unit 400 is supported on the base 100 by a rotary bearing 402 during the CMP process.
The load cup 300 is detailed in FIG. 1 and includes a pedestal support column 312 that supports a circular pedestal 310 on which the wafers are placed for loading of the wafers onto the polishing pads 210a, 210b and 210c, and unloading of the wafers from the polishing pads. A pedestal film 313 is typically provided on the upper surface of the pedestal 310 for contacting the patterned surface (the surface on which IC devices are fabricated) of each wafer. Fluid openings 314 extend through the pedestal 310 and pedestal film 313. The bottom surfaces of the polishing heads 410a, 410b, 410c and 410d and the top surface of the pedestal film 313 are washed at the load cup 300 by the ejection of washing fluid through the fluid openings 314.
In typical operation of the CMP apparatus 90, each wafer is mounted on a polishing head 410a, 410b, 410c or 410d and sequentially polished against the polishing pads 210a, 210b and 210c, respectively. This is shown in FIG. 2, wherein S1 indicates the first polishing step on the polishing pad 210a; S2 indicates the second polishing step on the polishing pad 210b; and S3 indicates the third polishing step on the polishing pad 210c. After polishing, the wafer may be subjected to cleaning, as indicated in step S4, followed by in-line metrology, as indicated in step S5.
The in-line metrology step (S5) frequently reveals that the polished wafers require additional polishing steps to remove additional material therefrom. Accordingly, many wafers subjected to CMP require a second series of polishing steps following the in-line metrology step. This fine polishing process is implemented according to the results of the metrology step to achieve a material layer target thickness that is optimal for further processing.
Although the actual material removal rate of the fine polishing process is about ⅙.about. ⅛ that of the first polishing process, the total process time is about the same as that of the first polishing process. This dual-processing of wafer lots substantially prolongs the process cycle time, reducing both equipment availability and process throughput. Accordingly, a CMP apparatus and process sequence method is needed which facilitates the in-line metrology of wafers prior to the third polishing step in a polishing process in order to obtain a material layer of target thickness without the need for an additional process cycle.
An object of the present invention is to provide a new and improved CMP apparatus in which a metrology tool is interposed between successive polishing pads in the apparatus.
Another object of the present invention is to provide a new and improved CMP apparatus in which a first metrology tool may be interposed between first and second polishing pads and a second metrology tool interposed between second and third polishing pads on the apparatus.
Still another object of the present invention is to provide a new and improved CMP apparatus and process sequence method which expedites the polishing of multiple wafers.
Yet another object of the present invention is to provide a new and improved CMP apparatus and process sequence method which substantially enhances wafer throughput in a CMP process.
A still further object of the present invention is to provide a CMP process sequence method according to which a wafer metrology step is interposed between successive polishing steps in a CNP process to facilitate optimal polishing of multiple wafers using a minimum number of polishing steps.